1. Field of the Invention
The present invention relates generally to wire-grid polarizers in the visible spectrum to correct a visible light beam or compensate for another optical element.
2. Related Art
When certain optical elements are exposed to plane polarized light, they cause changes in the polarization state. Short of complete depolarization, they can rotate the plane of polarization, induce some ellipticity in to the beam, or both. Such changes can occur uniformly over the beam of light, or it may only occur in certain portions of the beam. In any case, the resulting beam cannot be effectively extinguished by another linear polarizer which may be required in the optical train (e.g. to generate image contrast in a liquid crystal projection display). One solution is to put a “clean up” polarizer behind the element to reject light of the wrong polarization. Unfortunately, this dims portions of the transmitted light beam and may not be sufficient to restore sufficient contrast across the entire beam of light. The reduction of intensity, and especially the inhomogeneity of intensity and/or contrast across the beam is objectionable in many applications, and especially in imaging systems.
As an example, consider a spherical lens that is not dichroic or birefringent. Such a lens rotates polarized light by the following mechanism. The ray along the axis of the lens is un-deviated in its path, and completely maintains its polarization. Other rays will have their path changed by the action of the lens, causing a rotation of some degree in the polarization orientation of this ray. As a result, the light exiting the lens will have some rays which have maintained their polarization orientation, and other rays with rotated polarization orientations. It would be desirable to correct these polarization aberrations.
There are several types of polarizers:
Birefringent crystal prism polarizers are typically as long as they are wide (approximately cubic). They are made of polished, carefully oriented crystal prisms. As a result, they are expensive, and will polarize light only if it has very low divergence or convergence.
The MacNielle cube polarizer is not made of birefringent materials, but it is similar to crystal polarizers in many respects. For both of these, thickness, low acceptance angle and cost prohibit their effective use.
Thinner polarizers can be made of oriented, treated polymer sheets. Although they transmit most of the light of one polarization, they typically absorb virtually all of the light of the orthogonal polarization. This can lead to severe heating in intense light, and the polymers typically degrade at temperatures less than 200 degrees C. Because the absorbing particles are dispersed in the polymer, a certain thickness (approximately 0.05 mm) is required for adequate absorption of the unwanted polarization. In addition, the polymer material is not very stable in environments where temperature and humidity change frequently.
It has been proposed to make a more heat-resistant polarizer by orienting prolate metal spheroids embedded in glass provided the spheroids have dimensions that are small compared to the light to be polarized. Unfortunately, such polarizers can be difficult to produce. For example, see U.S. Pat. No. 5,122,907.
Another type of polymer based polarizer contains no absorbers, but separates the two polarizations with tilted regions of contrasting refractive indexes. The light enters from the open side of the V-shaped film, is reflected from one side to the other, and then out. For this retro-reflecting polarizer to work, both sides of the “V” must be present. They are of moderate thickness, do not resist high temperatures, and have limited angular aperture. Again, such polarizers are not easily produced. For example, see U.S. Pat. No. 5,422,765.
A heat-resistant polarizer can be made of inorganic materials of differing refractive index. Such polarizers can be thin (about 0.1-10.0 μm) because they are inhomogeneous films deposited at an angle on a substrate which may be thin. Unfortunately, there is considerable randomness to the placement of the transparent oxide columns that are deposited to provide the anisotropic structure for the polarizer. The randomness limits performance, so transmission is only about 40%, and the polarization is only about 70%. This optical performance is inadequate for most applications. For example, see U.S. Pat. No. 5,305,143.
Another evaporated thin film polarizer also is inefficient because of randomness. This type of polarizer is made by oblique evaporation of two materials, at least one of which is birefringent. For example, see U.S. Pat. No. 5,245,471.
Many of the above polarizers either absorb the orthogonal polarization, or reflect it in directions where it is difficult to use.